In optical communication, optical signals are communicated through a transport medium in order to communicate information. Optical communication allows a great deal of information to be communicated over a single transport medium, for example over a single optical fiber. Unfortunately, non-idealities in the properties of a transport medium, such as temperature sensitivity, humidity sensitivity, physical deformation, discontinuities and a finite spectral bandwidth, can cause dispersion of an optical signal travelling through the transport medium. All forms of dispersion degrade an optical signal, reducing the data-carrying capacity through pulse-broadening.
One form of dispersion is chromatic dispersion (CD), which results from a variation in group delay with wavelength, and is affected by transport medium materials and dimensions. In optical fiber, the two primary mechanisms for chromatic dispersion are material dispersion and waveguide dispersion. Both of these mechanisms occur because all optical signals have a finite spectral width, and different spectral components will propagate at different speeds along the length of the fiber. Material dispersion results from the index of refraction of the fiber core being different for different wavelengths. Material dispersion is often the dominant source of chromatic dispersion in single-mode fibers. Waveguide dispersion results because the cross-sectional distribution of light within the fiber also changes for different wavelengths. Shorter wavelengths are more completely confined to the fiber core, while a larger portion of the optical power at longer wavelengths propagates in the cladding of the fiber. Since the index of refraction of the core is greater than the index of refraction of the cladding, this difference in spatial distribution causes a change in propagation velocity and hence group delay. Waveguide dispersion is generally relatively small compared to material dispersion. In single mode fiber, the fiber dimensions and properties are generally designed such that the waveguide dispersion effectively cancels out the material dispersion for a narrow band of channel wavelengths. However, in these fibers the waveguide dispersion only effectively cancels out the material dispersion under a narrow range of operating conditions. For example, physical deformation of the fiber, fluctuations in temperature and/or humidity may cause the properties of the fiber to change such that the waveguide dispersion and material dispersion no longer effectively cancel each other for the narrow band of channel wavelengths.
Another form of dispersion is polarization mode dispersion (PMD), which results from a phase delay between polarization states of an optical signal. Single-mode optical fiber and components support one fundamental mode, which generally consists of two orthogonal polarization modes. Ideally, the core of an optical fiber is perfectly circular, and therefore has the same index of refraction for both polarization states. However, mechanical and thermal stresses introduced during manufacturing, installation or by the operating environment result in asymmetries in the fiber core geometry. This asymmetry introduces small index of refraction differences for the two polarization states, which is a property called birefringence. Birefringence creates differing optical axes that generally correspond to a fast and slow axes. Birefringence causes one mode to travel faster than the other, resulting in a difference in the propagation time called the differential group delay (DGD).
In some cases, stress rods may be placed in the cladding of a single mode fiber to place stress on the fiber core such that one polarization plane is favoured over the other in order to limit the transmission to only one of the two polarization modes. This type of single mode fiber is known as polarization maintaining fiber, however, environmental stresses including thermal stresses and mechanical stresses can still cause deformation of the fiber, possibly negating the intended effects of the stress rods.
When power is exchanged among the propagating polarization modes, an effect which is known as mode coupling, both the polarization modes and the DGD are also wavelength dependent. Mode coupling is generally only present in long lengths of single-mode fiber, however it is also sometimes observed even in short optical components.
PMD effects resemble those of chromatic dispersion. However, chromatic dispersion is a linear effect that is generally rather stable, whereas, PMD is a linear effect that is time-varying.
When designing an optical network, designers must allow a dispersion margin that takes into account the amount of dispersion that will arise in the network. If the amount of dispersion exceeds the margin, the dispersion must be compensated for.
Relatively complicated schemes have been described which only dynamically compensate for PMD. For example, conventional dispersion compensation schemes have used dispersion shifted fibers and/or fiber bragg gratings, but have not taken into consideration dispersion caused by both chromatic dispersion and PMD and have similarly failed to consider changes in the dispersion over time due to environmental fluctuations and mechanical stresses.